Photochemotherapy, or photodynamic therapy (PDT) as it is also known, is a technique for the treatment of various abnormalities or disorders of the skin—or other epithelial organs or mucosa, especially cancers or pre-cancerous lesions, as well as certain non-malignant lesions for example skin complaints such as psoriasis. Photochemotherapy involves the application of photosensitizing (photochemotherapeutic) agents to the affected area of the body, followed by exposure to photoactivating light in order to activate the photosensitizing agents and convert them into cytotoxic form, whereby the affected cells are killed or their proliferative potential diminished.
A range of photosensitizing agents are known, including notably the psoralens, the porphyrins, the chlorins and the phthalocyanins. Such drugs become toxic when exposed to light.
Photosensitizing drugs may exert their effects by a variety of mechanisms, directly or indirectly. Thus for example, certain photosensitizers become directly toxic when activated by light, whereas others act to generate toxic species, e.g. oxidizing agents such as singlet oxygen or other oxygen-derived free radicals, which are extremely destructive to cellular material and biomolecules such as lipids, proteins and nucleic acids. Psoralens are an example of directly acting photosensitizers; upon exposure to light they form adducts and cross-links between the two strands of DNA molecules, thereby inhibiting DNA synthesis. The unfortunate drawback of this therapy is that unwanted mutagenic and carcinogenic side effects may occur.
This disadvantage may be avoided by selecting photosensitizers with an alternative, indirect mode of action. For example porphyrins, which act indirectly by generation of toxic oxygen species, have no mutagenic side effects and represent more favourable candidates for photochemotherapy. Porphyrins are naturally occurring tetrapyrroles that are precursors in the synthesis of heme. In particular, heme is produced when iron (Fe3+) is incorporated in protoporphyrin IX (PpIX) by the action of the enzyme ferrochelatase. PpIX is an extremely potent photosensitizer, whereas heme has no photosensitizing effect.
One such porphyrin-based drug, Photofrin, has been approved as a photosensitizer in the therapy of certain cancers. A considerable disadvantage is that since it must be administered parenterally, generally intravenously, it can cause photosensitization of the skin which may last for several weeks following injection. Photofrin consists of large oligomers of porphyrin and it does not readily penetrate the skin when applied topically. Similar problems exist with other porphyrin-based photosensitizers such as Foscan (temoporfin) or the so-called “hematoporphyrin derivative” (Hpd) which have also been reported for use in cancer photochemotherapy. Hpd is a complex mixture obtained by treating haematoporphyrin with acetic and sulphuric acids, after which the acetylated product is dissolved with alkali. Foscan is a tetrapyrrole derivative with four meta-phenol groups attached, that after intravenous injection leaves the patient hypersensitized up to 3 weeks due to the very slow clearance rate from the body.
To overcome these problems, precursors of PpIX have been investigated for photochemotherapeutic potential. In particular the PpIX precursor 5-aminolevulinic acid (5ALA) has been investigated as a photochemotherapeutic agent for certain skin cancers.
Photodynamic therapy based on topical application of 5-aminolevulinic acid (5ALA) or a derivative thereof, for the treatment of small solid tumors is based on using the body's own biosynthetic route to form the endogenous chromophore protoporfyrin IX (PpIX) [1,2].
In Photodynamic diagnosis (PDD), the strong fluorescence of the chromophore is utilized. Excitation at 400-410 nm yields strong emission in the range 630-640 nm, enabling detection of the tissue in which PpIX is accumulated.
5ALA, which is formed from succinyl CoA and glycine in the first step of heme synthesis, is to a limited extent able to penetrate the skin and lead to a localised build-up of PpIX; since the action of ferrochelatase (the metallating enzyme) is the rate limiting step in heme synthesis, adding an excess amount of exogenous 5ALA (or a derivative thereof) bypasses the natural regulatory mechanisms, and leads to elevated levels of the photosensitizing agent PpIX in the cells, with a notable accumulation in tumorus cells [3,4]. A contributing factor for this is that the final enzyme in heme biosynthesis, ferrochelatase, that incorporates an iron ion into the PpIX ring system, is downregulated in tumor cells, thus leading to enhanced build-up of PpIX levels in cancer cells relative to normal cells.
In PDT, excitation of PpIX at 632 nm is generally used. The singlet excited chromophore undergoes efficient intersystem crossing to the first excited triplet state (T1). In presence of molecular oxygen, the chromophore passes its excitation energy from the T1 state to oxygen, thereby generating highly cytotoxic singlet oxygen. As PpIX is synthesized in the cellular mitocontria, the formed singlet oxygen attacks mitochondrial membranes with high efficiency, thus destroying the cells' capacity to produce energy, whereby the cell in question dies.
By applying 5ALA topically to skin tumors, and then after a few hours exposing the tumors to light, a beneficial photochemotherapeutic effect is obtained (see for example WO91/01727). Since the skin covering basilomas and squamous cell carcinomas is more readily penetrated by 5ALA than healthy skin, and since the concentration of ferrochelatase is low in skin tumors, it has been found that topical application of 5ALA leads to a selectively enhanced production of PpIX in tumors.
Photochemotherapy with 5ALA is not always entirely satisfactory. 5ALA is not able to penetrate all tumors and other tissues with sufficient efficacy to enable treatment of a wide range of tumors or other conditions and 5ALA also tends to be unstable in pharmaceutical formulations. These problems have to a large extent been overcome by the use of straight chain, unsubstituted alkyl 5ALA esters which exhibit improved selectivity for abnormal tissue, non-systemic localization of administered agents, improved uptake and PpIX production, and reduced pain sensation on administration (see WO96/28412).
The concentration of PpIX has been found to reach an optimal therapeutic window 2-4 h after application of 5ALA or derivatives thereof heretofore known and used [2]. The concentration of PpIX decays to normal background levels in 36-48 hours.
Currently, the PDT technique is employed using 5ALA or its methyl or hexyl ester clinically. Also other alkyl esters have been proposed [5].
Alkyl esters of 5ALA, and/or modifications to the amine group, are disclosed in for example U.S. Pat. No. 6,992,107 (family member of WO96/28412).
Recently a class of derivatized 5ALA esters, essentially comprising branched alkyl 5ALA esters and substituted benzyl 5ALA esters were proposed therein, providing advantageous enhanced PDT properties compared to the compounds mentioned above (U.S. Pat. No. 6,992,107; Gierskcky et al).
However, these compounds still exhibit some limitations for use as pharmaceuticals in PDT, e.g. relatively low efficacy of membrane penetration. Such a slow penetration hampers efficient clinical treatment by requiring longer retention times before irradiation, and may negatively influence the acceptance of treatment, both by the patient and the medical profession. Consequently, there exists a need for improved photochemotherapeutic agents.